This invention concerns high frequency resonators, also known as horns, mechanical amplitude transformers, concentrators, etc. operating in the ultransonic frequency range and used for welding or seaming thermoplastic workpieces, such as superposed layers of sheet material. More specifically, this invention refers to the construction of a high frequency resonator used in conjunction with an apparatus for welding together layers of sheet material instead of sewing them. Apparatus used to vibration weld, preferably by ultransonic vibrations, superposed layers of thermoplastic sheet material in a predetermined pattern are well known in the art, see for instance U.S. Pat. No. 3,733,238 issued to D. D. Long et al dated May 15, 1973, entitled "Apparatus for Vibration Welding of Sheet Material".
The patent to Long et al explains the need to have an ultrasonic apparatus constructed to weld the entire width of materials passed through the apparatus. The problem encountered heretofore was that one could not construct an array of ultrasonic resonators spanning the entire width of the sheet material without the presence of gaps between the resonators. In order to avoid the gaps, Long et al disclose the use of two linear arrays of resonators wherein the second row of resonators fills the gaps of the first row of resonators. An arrangement for avoiding the need for two rows of resonators and using a single array of resonators is shown in U.S. Pat. No. 4,146,416 issued to H. S. Goldman dated Mar. 27, 1979 entitled "Apparatus for Vibration Welding of Material". This patent discloses blade-shaped resonators of a suitable configuration for causing two juxtaposed resonators to have interfacing surfaces. For instance, this patent shows resonators of trapezoidal and "T"-shaped cross-section to provide for interfacial relationships between laterally juxtaposed resonator surfaces in order to prevent the existence of gaps when passing sheet material through the apparatus.
Resonators of the last mentioned construction have a serious shortcoming in that the resonators are dynamically unbalanced about the lateral axis. Forces generated by the acceleration of an unbalanced resonator mass provide a couple which produces flexure motion thereby stimulating undesired flexural resonances. This phenomenon presents a serious problem in high gain blade-shaped resonators which are driven at a high mechanical amplitude. The undesirable mechanical stress condition resulting from the unbalanced resonator causes failure of the acoustical components of the apparatus.